The above analysis suggests that significant changes over time in the
continuum net flux of SWP camera images may result both from changes in
the intensity transfer function (ITF) compounded by limitations inherent
in the background determination algorithm BCKGRD. To estimate
the effects of these problems on the uncalibrated, ripple-uncorrected
net fluxes we have taken the net fluxes from the NEWSIPS MXHI files
in Table 2
and normalized them to a common effective exposure time. We summed the
net fluxes along each of the orders of all pixels with quality flags
with values -2 and computed regression lines with time through
these order-summed ``continuum"
fluxes. Our assumption in this analysis is that the star is not intrinsically
variable. Figure 10 shows the slopes of the regression coefficients
through these summed fluxes. Noting the
segmented decrease with of these fluxes in
Figs. 1 and
6, we split repeated the regression fits for data samples in the
time intervals 1979.1-1990.5 and 1990.5-1995.7. These fits probably give the
more meaningful information because they show the distinct quantitative changes
in the behavior of the net fluxes before and after 1990. The
relations in Fig. 10 give the slopes in units of fractional change
per year in the far-UV continuum fluxes for Sco. For comparison, the
slopes are also shown for the B0.5e star Cas, which has many spectra
distributed through the mission. This star's slight variability in the UV does
not affect these values appreciably.

If the gross fluxes were perfectly calibrated and background fluxes were
precisely extracted, the relations in Fig. 10 would collapse to
a single flat line having all zero values. The fact that they are not flat
is due to a combination of the flawed background solutions early and midway
in the IUE mission and to the ITF calibration for the SWP camera being made
only for a single epoch. The ``dip" in these relations in the region
1150-1250 represents the initial undercorrection of the background
flux we discussed in 3. The low zeropoint from this error leads
to excessive net fluxes in this wavelength range early in the mission.
Likewise, the sharp minor dip at the order containing Lyman or the
C IV 1548-1550 lines are due to enhanced percentage changes from
a zeropoint errors when the total flux in the order is dominated by strong
absorption lines. On the other hand, the displacement of the late- and
early-epoch curves
in rate of change is probably due to the change in the ITF properties across
the camera beginning in 1990. Reference to the time degradations of SWP
low-dispersion fluxes (Garhart 1992, 1997) reveals both some similarities
and differences. For example, as in the low-dispersion analyses, the curves
in Fig. 10 show local minima in the negative slopes at 1450
and 2000 and
a maximum at 1700. The average decline over the mission lifetime
(solid line in our figure) also agrees fairly well with the slopes from the
low-dispersion analysis. However, we suspect this is coincidental because
the changes in the slopes between early- and late-epoch high-dispersion
images are themselves fairly large. Reference to the surfaces in
Fig. 8 suggests the reason for this coincidental agreement.
These surfaces show that the
degradation of flux in the nulls is consistently smaller in the lower right
section of the surface, both where the short-wavelength end of the
low-dispersion spectrum falls and where the maximum of the blaze function of
the echelle orders is located.
Thus, the collapsed high-dispersion data as represented in
Fig. 8 registers an average across the surface in the direction
of echelle dispersion which is similar to the more localized low-dispersion
values. Finally, we remark that
the rise in the curves at 1150 is probably not due to
background errors because this region of the camera is well represented by
unilluminated background pixels. We believe a more likely alternative is that
after 1990 the slopes at the toe of the ITFs steepened for pixels in the
short-wavelength region of the SWP camera. This results in an artificial
enhancement of the net fluxes in the weak, short-wavelength continua of
these orders.

Although we cannot yet recommend the relations given in this diagram
for use with datasets for other targets, it is possible that they may be
applicable to other early-type main sequence stars, particularly if the
exposure times are short as they are for Sco. For example, we have
compared these results with coefficients determined from 164
SWP high-dispersion, large-aperture images of the bright B0.5e star
Cas which were taken at regular intervals through the IUE lifetime.
A comparison of these, depicted as a dotted line in
Fig. 10, with
the Sco results demonstrates that the flux decreases in flux with
wavelength for images of these two stars are very similar, at least out to
the shortest-wavelength orders. We list in
Table 1 the values for
the dashed and dot-dashed curves ( Sco) of Fig. 10.
If one represents the fluxes
in an echelle order m at epochs 1979.1, 1990.5, and t during the
IUE mission as I1979(m), I1990(m), and It(m), respectively,
one may use the coefficients C1,t,(m) and C2,t,(m) from
Table 1
to adjust the net fluxes of spectra of
Sco to a
1979.1 reference frame by using two linear relations. The first is:

(1)

where the C's are the fractional loss of continuum net flux.
After 1990.5 one may correct to the 1979.1 reference frame by
a second linear relation:

(2)

In these equations is the time in years between the
observation at a time t and the appropriate earlier reference time, either
1979.1 or 1990.5. The loss of net fluxes relative to 1979.1 can be
corrected by means of eqn. 1 for images obtained during 1979-1990.
For post-1990 epochs, one first computes in eqn the flux for
= 10.4 years. This result, Im(1990), is used in
eqn 2 along with to compute the late-epoch flux
Im(t) relative to time 1979.1. Note that application of these
corrections to spectra of
Sco does not in any way modify or correct
spectral line depths. They represent a sensitivity correction for the
SWP camera to the NEWSIPS calibration.